This invention relates generally to electronic article surveillance (EAS) systems and, more particularly, to an apparatus and method for detecting in an EAS system a field which is subject to interference from one or more other fields which may be generated by other EAS systems operating in close proximity.
One form of EAS system presently known detects the presence of magnetic type tags which ar attached to articles which are under surveillance. This type of system is disclosed in U.S. Pat. No. 4,859,991, assigned to the same assignee hereof, and includes a transmitter which projects a magnetic field at a fundamental frequency into a surveillance zone which is monitored by a receiver. When an article carrying a magnetic tag is placed in the surveillance zone, the tag generates harmonics of the fundamental frequency which are detected by the receiver. The receiver then activates various alarms, or other appropriate signals, to indicate the presence of the tag and, therefore, the article in the zone.
In this type of system, large metal objects placed in the surveillance zone can, in some instances, generate harmonics similar to those produced by the magnetic tag. This can result in an inadvertent activation of the system alarm. To prevent this, the system is adapted to distinguish between tags and large metal objects.
More particularly, the receiver of the system is made to sense the amplitude of the magnetic field at the fundamental frequency projected by the transmitter. A change in this amplitude is recognized by the system as indicating the presence of a large metal object in the surveillance zone. Accordingly, upon detection of such change, the system inhibits the initiation of the system alarm, thereby avoiding false alarms due to the large metal object.
In an EAS system, once the transmitter and receiver are fixed in location, the amplitude of the fundamental magnetic field, i.e., the field at the fundamental frequency, in the surveillance zone will not vary appreciably over time, unless a large metal object is passed through the zone. Therefore, a single measurement of the amplitude of this field at initial set-up can be used as a baseline or reference value for detection o large metal objects during subsequent operation. More specifically, during such operation, the amplitude of the field measured at the system receiver is compared against the baseline. When a difference greater than a predetermined amount is detected, the EAS system determines that a large metal object is in the surveillance zone. It, therefore, enters an inhibit mode, whereby alarms are suppressed.
The above procedure of using the received amplitude of the system fundamental for detecting the presence of large metal objects in the system surveillance zone has worked satisfactorily where only a sole or first EAS system is present. However, where a second EAS systems is in close proximity to the first, the detection process is degraded. In particular, in such case, the first system's receiver detects the fundamental magnetic field in the surveillance zone resulting from both its own as well as the second system's transmitter. Since these fields are a result of different systems, they generally will not be totally synchronized in frequency and phase if they are not connected together.
As a result, the amplitude of the received fundamental magnetic field established in the zone as a result of the first system will be caused to vary over time based on the fundamental in the zone caused by the transmitter of the second system. Even if the transmitted fields are synchronized in frequency and phase, the received fundamental resulting from the first system still changes based on the on/off state of the second system. The presence of the second system thus causes changes in the received first system fundamental similar to those attributable to large metal objects in the surveillance zone. It, therefore, becomes difficult to determine the presence of such objects based on the detected first system fundamental. It may even be necessary to inhibit the suppression system, thereby increasing the susceptibility of the EAS system to false alarms due to large metal objects.
It is therefore a object of the present invention to provide an apparatus and method for determining the amplitude of a first field in a zone in the presence of a second field in such zone.
It is a further object of the present invention to provide an apparatus and method for use in improving the ability of an EAS system to distinguish between a field in a surveillance zone established by the EAS system and another field in the zone established by a nearby system.
It is a further object of the present invention to utilize the method and apparatus of the preceding object to enable an EAS system to better sense large metal objects in the surveillance zone.